An oscillator provides a timing signal which is used by a telecommunications base station for communications. In certain telecommunications systems it is important that the timing signal provided by a base station is synchronized with timing signals in other base stations. A Global Positioning System (GPS) reference timing signal can be derived from GPS signals that are received by a GPS receiver in the base station to provide a uniform reference timing signal to which oscillators in base stations can be locked, enabling base stations to synchronize with one another.
An oscillator in a base station is responsive to a drive signal. Because the oscillator may drift over temperature and time, the drive signal must be corrected periodically to keep the oscillator locked to the GPS reference timing signal. During normal operation when the GPS reference timing signal is available, a primary correction signal is used to correct the drive signal. However, when the GPS signals are unavailable due to equipment failures or environmental conditions, for example, an auxiliary correction signal must be provided in place of the primary correction signal until the GPS signal becomes available again. This is sometimes referred to as a holdover period. If the auxiliary correction signal is inaccurate during the holdover period, the base station will lose synchronization with the other base stations. Unfortunately, generating accurate auxiliary correction signals has proven to be difficult, expensive, or both. As such, there is a need for an effective and efficient technique for generating accurate auxiliary correction signals when the GPS signals are unavailable.